The present invention relates to a semiconductor laser diode, and more particularly to a semiconductor laser diode based on InGap/InAlGaP which is capable of shortening the wavelength of visible ray lasers.
Lasers can be generally classified into solid state lasers, gas lasers, liquid lasers and semiconductor lasers in accordance with the medium used.
Also, lasers can be largely classified into ultraviolet rays lasers, infrared rays lasers and visible ray lasers in accordance with the wavelength of the output ray.
Also, semiconductor lasers can be largely classified into semiconductor lasers based on GaAlAs which are oscillating at a wavelength range of 0.7-0.9 .mu.m and used for communication utilizing optical fibers, semiconductor lasers based on InGaAsP which are oscillating at a long wavelength range of 1.1-16 .mu.m and semiconductor lasers which are oscillating at a short wavelength region of 0.7 .mu.m and are used as an emitting source for processing optical information of laser printers, compact discs and video optical discs, in addition to using such semiconductor lasers for communication.
In general, semiconductor laser diodes of visible wavelength are widely used as emitting sources of optical-magnetic discs and bar code readers and have an wavelength of about 670 nm. FIG. 1 shows a sectional view of a conventional semiconductor laser based on InGaP/InGaAlP.
Referring to FIG. 1, a n-type GaAs substrate 1 is prepared. Over the n-type GaAs substrate 1 is formed a n-type clad layer 2 which has a thickness of 2-3 .mu.m. The n-type clad layer 2 is made of In.sub.0.5 (Al.sub.0.7 Ga.sub.0.3).sub.0.5 P.
Over the n-type clad layer 2 is formed an active layer 3 which has a thickness of 0.1 .mu.m and is doped with no impurity ions. The undoped active layer 3 is made of Ga.sub.0.5 In.sub.0.5 P. Over the active layer 3, a p-type clad layer 4 having a thickness of 2-3 .mu.m is formed which is made of In.sub.0.5 (Al.sub.0.7 Ga.sub.0.3) .sub.0.5 P, similar to the n-type clad layer 2.
Over the p-type clad layer 4, a n-type GaAs layer is subjected to an etching process, thereby making a stripe cavity for confining current in the n-type GaAs layer. The n-type clad layer 2, the undoped active layer 3, the p-type clad layer 4 and the n-type GaAs layer are all formed by using a Molecular Beam Epitaxy method or Metal organic chemical vapour deposition method. The n-type GaAs layer having the stripe cavity serves as a current confinement layer 5. Over the current confinement layer 5, a p-type GaAs cap layer 6 is formed.
Finally, a p-type electrode 7 is formed at the upper surface of the p-type GaAs cap layer 6 and a n-type electrode 8 is formed at the lower surface of the n-type GaAs substrate 1. The semiconductor laser diode shown in FIG.1 has a double hetero (DH) structure in which the oscillating GaAs active layer 3 is positioned between the n-type clad layer 2 made of In.sub.0.5 (Al.sub.0.3 Ga.sub.0.7) .sub.0.5 P and the p-type clad layer 4 made of In.sub.0.5 (Al.sub.0.3 Ga.sub.0.7) .sub.0.5 P.
That is, according to a semiconductor laser diode having a DH structure, the GaInP active layer 3 having a small energy band gap is enclosed by the n-type InAlGaP clad layer 2 and the p-type InAlGaP clad layer 4 having an energy band gap larger than that, respectively.
FIG. 2a to FIG. 2c are energy band diagrams showing relations. between the n-type InAlGaP clad layer 2, the undoped InGaP active layer 3 and the p-type InAlGaP clad layer 4. The conventional semiconductor laser diode shown in FIG. 1 has an energy band structure shown in FIG. 2b under an equilibrium state.
As shown in FIG. 2c, if a high forward bias is applied to this semiconductor laser diode, electrons from the n-type clad layer 2 and holes from the p-type clad layer 4 are injected into the active layer 3.
As a result, energy barriers are formed by the n-type clad layer 2 and the p-type clad layer 4, thereby causing the injected electrons and holes to be confined in the active layer 3.
As shown in FIG. 2c, more carriers(electrons and holes) are distributed at a higher energy level than a lower energy level as a high forward bias is applied to the semiconductor laser diode.
This distribution is called an inversion distribution or a population inversion.
Therefore, an inversion distribution of electrons and holes is formed in the active layer 3 and electrons and holes are recoupled together.
At such an inversion distribution, light is inducibly emitted by virtue of the electron-hole recouping the inducible emission of light is called a stimulated emission.
However, since the active layer 3 of the conventional semiconductor laser diode having a DH structure is thickly deposited with a thickness of about 1 .mu.m, the containing-rate of Indum(In) in the active layer 3 should be restrained to 0.5, so as to achieve a lattice-matching.
As above mentioned, since the wave length is restrained to 670-690 nm, the conventional semiconductor laser diode has a disadvantage in that the quantity of information to be recorded is limited when the conventional semiconductor laser is used to record information on an optical-magnetic disc.
So as to obtain a shorter wavelength of laser in the visible wavelength-semiconductor laser diode of FIG. 1 having a DH structure, a compound semiconductor such as InAlGaP may be used in place of a compound semiconductor such as InGaP, as a material of the active layer 3.
In this case, the wavelength of laser can be shortened up to 630 nm.
However, a band distance between the active layer 3 and the clad layer 2 .lambda. becomes narrow, thereby causing leakage current to be increased and moreover causing temperature characteristic to be deteriorated.